Hi,
You can do it by converting your file either with a lower bit rate or using an another compression algorithm. Be aware taugh that by doing so, you will loss audio quality. The best way is to restart, if possible, from your original files with your new importing settings.
To change your importing settings, go in the iTunes preferences, in the Advanced tab and then in the Importing tab and set the compressing algortihm and bit rate. AAC encoder is the algorith that will gives you the better compressing ratio.
Then, you you can either chose to reimport from your original CD (iTunes will ask you to replace the previously imported files) or select from the advanced menu the converting selection.